As described in certain embodiments of U.S. Pat. No. 5,518,964, the disclosure of which is incorporated by reference herein, an element used in a microelectronic assembly may have elongated flexible leads extending along a surface of the element. Each lead has a first or terminal end permanently attached to the element and has a tip end offset from the terminal end. A second element having contacts thereon is engaged with the first element so that the tip ends of the leads are bonded to contacts on the second element. The first and second elements are then moved away from one another so as to deform the leads and provide vertically extensive leads extending between these elements. A compliant material may be introduced between the elements during or after such motion. The resulting structure allows relative movement of the elements without substantial stresses on the leads. This process can be used, for example, to make packaged semiconductor chips. One of the elements may be a connection component desirably having a flexible structure incorporating one or more dielectric layers, and the other one of the elements may be a chip, wafer or assemblage of chips. The leads may be provided either on the connection component or on the chips or wafer.
As described in U.S. Pat. No. 5,763,941, the disclosure of which is also incorporated by reference herein, a releasable lead structure may be made by providing a first region of a lead permanently connected to a component and a second region releasably connected to a surface of the component so that the second region can be peeled away from such surface. Desirably, the second region is attached to the underlying surface by a release interface having a peel strength of less than about 0.35xc3x97106 dynes/cm; i.e., a peel strength of less than about 2 pounds per linear inch. For example, certain low-energy vapor deposition processes such as electroless plating or chemical vapor deposition can deposit a thin layer of release metal forming a release interface with the underlying layer. The lead may incorporate a structural metal which may be the same as or different from the release metal. The release metal can be peeled away from the underlying layer. In other embodiments, the interface may include a layer of a first release metal securely bonded to the underlying layer and a second release metal on the lead, the first and second release metals being weakly bonded to one another so that the second region of the lead can be pulled away from the underlying layer, with the first release metal remaining on the underlying layer.
Despite all of these improvements, still further improvements and variations would be desirable.
One aspect of the invention provides methods of making microelectronic assemblies. The method according to this aspect of the invention desirably includes providing a first element having a first surface and a plurality of leads extending over said first surface, the leads having anchor ends attached to the first element and tip ends remote from said anchor ends releasably attached to the first element. The method desirably further includes connecting the tip ends of the leads to a second element. The releasable attachment of the tip ends maintains the tip ends in position relative to said first element at least until commencement of said connecting step. After the tip ends of the leads have been connected to the second element, differential expansion or contraction of said elements is induced, most preferably by heating or cooling one or both of the elements. This differential expansion or contraction moves tip ends of the leads relative to the first element and breaks at least some of the releasable attachments. Breakage of the releasable attachment allows the tip ends of the leads to move relative to the first element. This breaking action frees the tip ends of the leads, so that the leads are free to flex during service of the finished device.
The releasing action caused by differential expansion or contraction can be combined with vertical movement of the elements away from one another to bend the leads to a vertically-extensive disposition. The releasing action caused by differential expansion or contraction avoids or reduces the need to break the tip ends away from the first element during vertical movement, and thus facilitates the vertical movement of the elements away from one another. In certain embodiments, the differential expansion or contraction of the elements can actually cause the vertical movement. As further discussed below, the elements can be linked by relatively strong restraining straps oriented so that the ends of the restraining straps will move in horizontal directions towards one another during differential expansion or contraction of the elements. Buckling of the restraining straps causes one end of each restraining strap to move with a vertical component of motion, thereby moving the elements away from one another. In other embodiments, buckling of the leads induces vertical movement of the elements away from one another.
Where the first and second elements have different coefficients of thermal expansion, the step of inducing differential expansion or contraction may include altering the temperature of both of the elements. The connecting step can include bonding the tip ends of the leads to said second element while both of said elements are at an elevated temperature, and the step of altering the temperature of both of said elements may include cooling these elements.
In certain preferred embodiments, one of the elements includes one or more semiconductor chips and the other one of the elements includes a connection component having oppositely-directed inner and outer surfaces and terminals exposed to the outer surface. The connecting step is performed so that the inner surface of the connection component faces toward the one or more semiconductor chips, and the outer surface of the connection component faces away from said one or more semiconductor chips and so that at least some of said leads are electrically connected to at least some of said terminals. Thus, the chips are connected to the terminals. Preferably, a flowable material is injected between the one or more semiconductor chips and the connection component after the connecting step. The flowable material is cured to form a layer surrounding the leads. The resulting structure provides one or more packaged semiconductor chips having terminals connected to the contacts on the chip, the terminals being movable relative to the chips.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments, set forth below, taken in conjunction with the accompanying drawings.